Total knee prosthesis with ceramic-on-ceramic friction torque and mobile ceramic plate

ABSTRACT

A total knee prosthesis to be implanted in a human patient includes a femoral element having a longitudinal axis, a tibial plateau having a longitudinal axis and a mobile plate. The mobile plate is interposed between the femoral element and the tibial plateau to form two joints with them wherein: a) the surfaces of mutual friction of the femoral element with the mobile plate and the surfaces of mutual friction of the tibial plateau with the mobile plate are entirely constituted by one and the same massive ceramic material; and b) the mobile plate includes two condylar bowls, and the femoral element includes two condyles, the condyles and the condylar bowls each having surfaces of mutual friction spaced apart from each other by a distance smaller than 100 μm when the longitudinal axes of the femoral element and the tibial plateau form an angle of 0° to 75°.

1. CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a continuation of U.S. application Ser. No.15/549,305, filed Aug. 7, 2017, which is a Section 371 National StageApplication of International Application No. PCT/EP2016/052486, filedFeb. 5, 2016, published as WO 2016/124731 on Aug. 11, 2016, not inEnglish, the contents of which are incorporated herein by reference intheir entireties.

2. FIELD OF THE INVENTION

The field of the invention is that of joint prosthesis.

More specifically, the invention relates to a total knee prosthesis madeof ceramic, comprising a mobile ceramic plate to replace a patient'sknee joint.

3. PRIOR ART

Knee joint replacement is a frequent surgical procedure. It is estimatedthat, in 2011, about 650,000 prostheses were implanted in the UnitedStates and 80,000 in France. Knee prostheses can be total or partial:unicondylar, total and then hinged knee prostheses or sled kneeprostheses.

Sliding (or sled) prostheses comprise a femoral condylar element and atibial baseplate, respectively attached to the profiled extremities ofthe femur and tibia. The majority of prostheses implanted are what arecalled “sliding” prostheses. These sliding prostheses comprise a femoralpart made of metal or ceramic and a tibial part made of polyethylene,generally embedded in the tibial baseplate made of metal. Certain ofthese prostheses have a mobile plate made of polyethylene which can moveon the tibial plateau. The theoretical advantage of the mobile platethat it reduces wear on the polyethylene by enabling more consistentadaptation of the friction surfaces and improving knee flexion when themobile plate is moved not only rotationally but also from front to rear(“drawer” movement), the mobile plate moving on the tibial surface likea meniscus.

These sliding prostheses do not replace the cruciate ligaments of theknee and must be implanted perfectly in keeping with the naturalarchitecture of the surfaces of the knee and of the anatomical axis.Certain prostheses keep the posterior cruciate ligament while others donot. There is then a central cap that prevents the tibia from fallingrearwards.

Until now, total prostheses have been made of metal and polyethylene.According to this technique, the femoral and tibial baseplates are madeout of a metallic material while a polyethylene pad is mounted fixedlyor movably on the tibial baseplate. Other embodiments consist of afemoral base made of ceramic cooperating with a polyethylene plate,mounted on a tibial baseplate made of ceramic or metal. When theprosthesis is designed with a mobile plate, the mobility of the mobileplate can be limited by the shape of the metal receptacle fixed into thetibia which permits only rotational movements or it can be limited by acentral nipple which, depending on the shape of a lower cavity designedin the mobile plate, permits rotational and drawer movements.

However, these materials cannot be used to design a joint prosthesisthat is robust enough to allow the patient to resume normal physicalactivity: while a knee prosthesis makes it easy to resume painlesswalking, patients generally feel some discomfort during prolongedwalking or when walking up or down stairs. Besides, running or any othermajor physical activity remains prohibited.

Now, the average age of patients is dropping considerably. Many surgicaloperations indeed are a consequence of sports-related or work-relatedaccidents among young and professionally active people. It is thereforedifficult and particularly frustrating for people to have to give up anyform of physical activity or even their professional activity.

In addition, the failure rate of these prostheses is currently 10% atthe end of the first 10 years. The rate rises to 25% among youngerpatients. The failure of the prosthesis means that another operation hasto be performed on the patient in order to replace his prosthesis. Nowthese repeat surgical operations entail, on the one hand, a great dealof suffering for the patient and, on the other hand, a considerable costfor health insurance systems. They also represent a legal risk fornursing homes and hospitals.

However, the main cause of these failures lies chiefly in the in thewearing out of the polyethylene constituting friction torque. Therelease, under mechanical stresses, of wear particles causes aninflammatory reaction that is responsible for joint effusion as well asthe creation of a granuloma of periarticular resorption. Bone lysis inthose parts of the femur and/or the tibia that neighbor the prosthesishas been observed, possibly resulting in major bone loss. Thisosteolysis can also lead to the loosening of the prosthesis. When thewear is very great, the metal parts can be bared, leading to reactionsof massive rejection through the production of toxic metal particles(metallosis).

There is the known document WO-2011/003621-A1 which especially describespartially ceramic knee prosthesiss as well as a wear resistant devicefor these prostheses. However, no prosthesis according to this techniquehas appeared to date on the market. This is the case for the followingreasons.

First of all, the prostheses according to this document comprise a metalrod to which a tibial baseplate is fixed. This leads to high abrasion ofthe metal. The abrasion of the metal results in the formation of metaldebris in the body. The organism reacts against these metal particles byproducing an inflammatory reaction like that described with polyethyleneparticles.

In addition, affixing the ceramic parts to a metal rod raises problems:it is in practice very difficult to affix this type of element withoutcreating conditions of breakage of the ceramic. Besides, the document WO2011/003621 A1 describes no method for affixing these parts. Inaddition, although the mobile plate described is made of ceramic, thewide oval aperture at the center of the mobile plate is in practiceimpossible to make in a material using massive ceramic since this shapeof aperture and its dimensions make the ceramic brittle. Similarly, theimpact of the aperture made in the ceramic plate against the stops 18 issuch as to induce premature fractures of the ceramic plate, if thisplate were to be made of massive ceramic.

One solution for countering these drawbacks is to propose ceramic-coatedmetal parts. However, the fine layer of ceramic on the surface of themetal gets worn out and the metal can be bared, prompting theinflammatory reactions referred to here above. This practice has not forthe time being shown any sort of superiority.

It is therefore necessary to design total knee prostheses that are morewear resistant, so that the wear particles produced do not causeinflammatory reactions, thus enabling the patients to return to normallife. It is also desirable that the prosthesis or its debris shouldproduce a dense fibrous tissue capable to stabilizing the knee in a waythat ligaments would do.

4. SUMMARY OF THE INVENTION

An aspect of the present disclosure relates to a total knee prosthesisintended for being implanted in a human patient, said prosthesiscomprising a femoral element having a longitudinal axis, a tibialplateau having a longitudinal axis and a mobile plate, said mobile platebeing interposed between the femoral element and the tibial plateau toform two joints with them wherein:

a. the surfaces of mutual friction of the femoral element with themobile plate and the surfaces of mutual friction of the tibial plateauwith the mobile plate are entirely constituted by one and the samemassive ceramic material; and

b. the mobile plate comprises two condylar bowls, and the femoralelement comprises two condyles, said condyles and said condylar bowlseach comprising surfaces of mutual friction spaced apart from each otherby a distance of less than 100 μm when the longitudinal axes of saidfemoral element and said tibial plateau form an angle of 0° to 75°.

This distance enables perfect congruency between the surfaces of mutualfriction especially in a load-bearing area (for example in a staticstanding position or in movement).

In the following description of the invention, the longitudinal axis ofthe femoral element essentially coincides with the axis of the femur ofthe patient in whom the prosthesis will be implanted. Similarly, thelongitudinal axis of the tibial plateau essentially coincides with thelongitudinal axis of the tibia of the patient receiving the prosthesis.The term “essentially coincides with” is understood to mean that the tworeference axes form an angle of less than 5° with each other,alternatively less than 2°, and alternatively again less than 1°, andalternatively again an angle equal to 0°.

Thus, the prosthesis according to the invention comprises three ceramicelements made of massive alumina ceramic intended for being hingedrelatively to one another, along two joints with congruent surfaces. Thefemoral element fits into the prepared distal extremity of the femur andreproduces the shape of the trochlea. It is therefore necessary to havea right femur element and a left femur element. The tibial plateau isanchored in the upper extremity of the tibia that is prepared to receiveit. Finally, the mobile plate is situated between the femoral elementand the tibial plateau to form a joint. The invention therefore relieson a wholly novel and original approach to designing a total bicondylarknee prosthesis in which all the surfaces in contact with the elementsthat form it are made out of a same ceramic material. This favors areduction in the quantity of wear debris inasmuch as the designing ofthe parts with two distinct joints makes it possible to preserve closecontact, in all degrees of flexion of the knee, between the surfaces,this being a major factor for reducing the quantity of debris formed.This contact or congruency is obtained both in the load-bearing area andin the flexion area.

Unlike in current prostheses which comprise a femoral element made ofmetal or ceramic, in contact with a fixed or mobile plate made ofpolyethylene, the prosthesis according to the invention comprises amobile plate made of a massive ceramic material that is the same as thatof the femoral element.

According to the invention, the femoral element, the mobile plate andthe tibial plateau are entirely constituted out of one and the sameceramic material. In other words, the femoral element, the mobile plateand the tibial plateau are all three made of massive ceramic, whichmeans that they do not include any alloy with plastic material or metalmaterial such as polyethylene, titanium, metalized ceramic etc., or thatthey are not constituted by a fine layer of ceramic fixed to a metal orplastic element.

Until now, to the knowledge of the present Applicant, it has never beenproposed to place two ceramic parts in close contact to design a kneeprosthesis. There have been known ways of proposing ceramic hipprostheses among young and/or active individuals. However, themechanical stresses exerted on the knee joint are very different fromthose exerted on the hip, since the hip and knee joints have verydifferent geometries. The use of ceramic to make a knee prosthesis inwhich the friction surfaces of the tibial plateau and the femoralelement are made entirely out of ceramic has always been rejected byprosthetic specialists and orthopedic surgeons, for various reasons. Thefirst reason is that the mechanics of the knee is complicated. Duringflexion, there are successive phases of rolling and sliding. Thesephases are tolerated by a pair of materials constituted by hard/softmaterials but cannot be envisaged with a pair of hard/hard materials,since this latter pair does not work satisfactorily at the tribologicallevel unless high congruency persists between the friction surfaces.

The second reason is that ceramics is a material with brittle behaviorand can break in the propagation of a crack.

Apart from the health risk for a patient, cases of breakage ofprosthesis entail costs and major legal risks for health professionals.This must be compared with the tenfold risk of repeat surgery due to theconsequences of reactions to wear debris in prostheses containingpolyethylene.

Another prejudice against the use of ceramic in total knee prosthesiscomes from the possibility that mutual friction between the two ceramicparts will produce a scratching sound at each movement, this sound beingheard by the patient and sometimes even by people surrounding him. Thisphenomenon has already been observed in certain hip prostheses and canbe a source of discomfort in daily life for the patient. The designingof parts as well as the interposing of metal particles is the factormost frequently implicated in this problem. The invention prevents thisdrawback by providing for a sufficient thickness of ceramic and byeliminating the ceramic/plastic or ceramic/metal surfaces of mutualfriction. Besides, the parts of the prosthesis according to theinvention can be fixed into the bone with an acrylic cement that dampensvibrations.

Preferably, said ceramic material has a thickness of 4 mm to 14 mm. TheApplicant has indeed noted that ceramic prostheses having thicknessesoutside this range, are brittle. This is not desirable. In particular,prosthetic prototypes have been proposed wherein a fine layer of ceramicis attached to the metal part. Such prostheses are extremely brittle andare therefore not used in the form of ceramic/ceramic friction elements.

Another advantage of ceramic is that it is better tolerated by theorganism than metal or polyethylene. The advantage of ceramic-on-ceramicfriction is that it enables the recreation of fibrous tissue or a“neo-ligament” acting as a natural ligament after adaptive remodeling.Indeed the formation of a fibrous tissue is observed. This fibroustissue is more resistant and of better quality with the prosthesesaccording to the invention than it is with the usual prostheses in whichthe friction surfaces are constituted by metal against polyethylene orceramic against polyethylene. The wear particles generated in smallquantities during the movements of the joint do not give rise tomacrophage reaction in the organism. The prostheses of the inventiontherefore avert the problem of osteolysis and the risks of dislocationof the prosthesis observed with present-day prostheses.

Quite to the contrary, the Applicant has observed that small quantitiesof alumina particles give rise to a particularly dense fibrous tissue,even denser than that formed around current prostheses. This fibroustissue plays a role in reinforcing and stabilizing the joint capsule andreplacing the ligaments. The prosthesis of the invention thereforeenables patients to have a knee joint that is more stable and resistantto forces. This particular feature therefore enables patients to keeptheir prosthesis for a longer time and to recover normal physicalactivity.

The Applicant has furthermore noted that the behavior of two ceramicparts against each other gives a prosthesis that is highly resistant andmore robust than existing ones, contrary to the prejudices listed hereabove. These special technical advantages are due especially to perfectcongruency between the friction surfaces, which reduces friction betweenthe parts and therefore the risk of wear in the prosthesis.

As understood in the invention and in the following description, theperfect congruency of the parts relative to each is obtained when theparts perfectly match each other, i.e. with very limited clearancesbetween the surfaces in contact, and when there is no area of thesurfaces of friction of the parts relative to each other in which themechanical stresses are concentrated. This perfect congruency can alsobe qualified as being ideal in the description.

This perfect congruency or ideal congruency of the surfaces of mutualfriction is obtained through the choice of simple shapes for thesurfaces: the condylar/condylar bowl surfaces are portions of cylindersand/or spheres having the same radius of curvature and the surface ofcontact between the mobile plate and the tibial plateau is plane. Theseshapes are obtained by specific manufacturing and honing techniques,mastered by the industry specializing in ceramic prostheses. To put itbriefly, the profiles of the different parts produced are evaluated bymeasuring the radii of curvature of the different parts after theseparts are subjected to different wear tests. The goal of thesemeasurements is to obtain a spacing between the parts and morespecifically between the mobile plate and the femoral element.

According to the invention, the space between the surfaces of mutualfriction of the femoral element and the mobile plate ranges from 0 μm to100 μm, preferably from 10 μm to 60 μm, when the longitudinal axes (L,L′) of said femoral element and said tibial plateau form an angle of 0°to 75° with the longitudinal axis of the mobile plate.

Advantageously, the space included between the surfaces of mutualfriction of the mobile plate and the tibial plateau also ranges from 0μm to 100 μm, preferably from 10 to 100 μm, and even more preferablyfrom 10 μm to 60 μm.

Selecting the distance between the surfaces of mutual friction is ofprime importance. Indeed, the inventors have shown surprisingly that, inorder to ensure optimal congruency without impairing the comfort and theoperation of the prosthetic joint, this distance should preferably notbe less than 10 μm and more than 100 μm, preferably not more than 60 μm.Indeed at a distance of less than 10 μm, there is a risk that the liquidwill not flow sufficiently. If is over 100 μm, there is a risk ofconcentration of stresses on one part of the prosthesis that will becomeexcessively high.

For all these reasons, the prostheses according to the invention aremore resistant, and limit or even prevent the drawbacks observed withpresent-day prostheses in which the mobile plate is made ofpolyethylene. The present invention thus enables patients to return tonormal life with a greater level of physical activity than that allowedto them at the present time. It is no longer necessary to carry outrepeat surgery to replace used prostheses or, at the very least, thedate of the second operation is considerably later than with present-dayprostheses.

Until now, it was considered to be impossible to design a total kneeprosthesis in which the elements forming the joint—i.e. the femoralelement, the mobile plate and the tibial plateau—were entirely made ofmassive ceramic because no adequate design was proposed for this. It wasindeed not possible, with existing designs, to maintain perfectcongruency during the totality of the movements, this congruency beingindispensable when the ceramic must be made to be in friction withitself. The particular design of the prosthesis according to theinvention around a mobile plate with a specific shape and the spacingplanned between the surfaces together eliminate these areas ofconcentration of mechanical stresses and therefore enable themanufacture of total knee prostheses in which all the elements formingthe joint are made of massive ceramic.

This is therefore a selection of particular specific characteristicsthat enable the designing and implementing of a total knee prosthesismade of ceramic.

Preferably, the ceramic material is an alumina ceramic (Al₂O₃) thatbrings about the synthesis of a fibrous tissue of better quality thanthat obtained with other ceramics. Such materials are of a quality fitfor surgery i.e. they must comply with prevailing standards on materialsfor the manufacture of surgical prostheses. For example, alumina ceramiccan be a dense polycrystalline ceramic obtained with aluminum oxidepowder compressed at temperatures of about 1600° C. through the HIP(High Isostatic Pressure) method. One ceramic suited to the implementingof the invention is a ceramic that complies with the ISO-6474-1standard.

Preferably, the ceramic has a grain size of less than 2 μm. Even morepreferably, the ceramic used to manufacture the prosthesis according tothe invention has a grain size of 0.5 μm to 2 μm. Even more preferably,the ceramic has a grain size of 1 μm to 2 μm. The grain size of theceramic material is determined according to any method well known tothose skilled in the art.

In one promising variant of the invention, the femoral element is alsocapable of cooperating with the patient's patella via a protrusion thatmimics a trochlea on the anterior part of the femoral element.

Advantageously, the tibial plateau is anchored by its lower face in theupper extremity of a patient's prepared tibia. A design taking the formof a massive rod that is integral with the tibial plateau is fixed intothe patient's bone by acrylic cement.

The dimensions of the parts constituting the prosthesis depend on thesize and dimensions of the joint to be replaced. For example, the mobileplate can have a maximum thickness of 4 mm to 14 mm. This ceramicthickness is sufficient to ensure stability of the knee while preventingthe risk of fracture of the ceramic.

Advantageously, said condylar bowls and said corresponding condyles arespaced apart from one another by a distance of less 100 μm, preferably adistance of 10 μm to 100 μm and more preferably a distance of 10 μm to60 μm, when the longitudinal axis of the femoral element andlongitudinal axis of the tibial plateau form an angle of 0° to 60°. Theangular range of 0° to 60° corresponds to the load-bearing area of theprosthesis. The load-bearing area is defined by the zone and amplitudeof flexion of the knee on which the joint supports the essential part ofthe patient's weight.

Advantageously, the femoral element and the tibial plateau are mobile inrotation relative to each other on an angular range of flexion of 0° to135°.

In other words, the longitudinal axis of the tibial plateau and thelongitudinal axis of the femoral element can form an angle of 0° to135°.

As understood in the invention, the mobile plate is mobile in aforward/rear, lateral and/or pivoting direction about a longitudinalaxis. Thus, the mobile plate of the invention is never solelyrotational.

This characteristic enables the patient receiving the prosthesis to befreer in his movements and especially in his ability to climb astaircase, bend his knee to crouch or kneel or simply have moreamplitude in his movements without feeling any block. The movementsallowed by the knee prosthesis of the invention are then very close tothe natural movements allowed by the knee. It is the movement proper toeach patient's knee that creates the positioning of the areas of newlyformed fibrous tissue and thus makes it possible, after a few weeks, toobtain the anticipated result. This amplitude of movement for example isnot allowed by the prosthesis described in the documentWO-2011/003621-A1, which cannot bend beyond 90°, thus preventing thepatient from going up a staircase for example and making it impossibleto propose the prostheses described in this document to a patient.

In one promising embodiment, the surfaces of mutual friction of thecondyles and of the condylar bowls are cylinder portions generated byrevolution and having a same radius R.

In another embodiment, the surfaces of mutual friction of the condylesand the condylar bowls are sphere portions having a same radius R′.

In another embodiment, one of the surfaces of mutual friction of thecondyles and the condylar bowls is a sphere portion having a radius R′and the other one is a cylinder portion generated by revolution having aradius R, the center of the sphere being on the axis of revolution ofthe cylinder.

The shape of the cylinder portion and/or sphere portion can beappreciated when we look at a sagittal section of the prosthesis, madeat about 20 to 27 mm from the median plane of the prosthesis accordingto the invention. Those skilled at the art will know how to easilydetermine the way to place the sagittal plane.

It must be noted that natural condyles generally have a spherical shapeformed by several successive spheres, the radii of which diminish fromfront to rear, the center of the spheres having an elliptic shape. TheApplicant herein proposes a shape that is slightly different from thenatural shape, making it possible to provide ideal congruency of theelements of the prosthesis relative to each other during the movement ofthe knee and therefore their ceramic embodiment. Indeed, the centers ofthe cylinder or sphere portions of the surfaces of friction between themobile plate and the femoral element are not on an ellipse but on anaxis.

In some variants, R and R′ can be equal. The dimensions of theprostheses and parts constituting them are easily determined by thoseskilled in the art. It is indeed common practice to propose differentsizes of prostheses to adapt to different sizes of patient operatedupon.

These three embodiments make it possible to design a prosthesis in whichthe mechanical constraints are not concentrated on a precise area, whichcould induce a crack in the ceramic.

Advantageously, said mobile plate comprises an intercondylar studforming a projection towards said femoral element, said femoral elementcomprising an intercondylar gap or empty space that houses saidintercondylar stud. Thus, the femoral element and the mobile platecooperate along a third series of surfaces of mutual friction.Preferably, the intercondylar gap and the intercondylar stud arecylinder portions or sphere portions. If the intercondylar stud and theintercondylar gap are cylinder portions, they are coaxial with thecylinder portions forming the condyles and condylar bowls. If theintercondylar stud and intercondylar gap are sphere portions, the centerof this sphere is situated:

-   -   either on the axis of revolution of the cylinder portions        forming the condyles and condylar bowls;    -   or on the axis connecting the centers of the sphere portions        forming the condyles and condylar bowls.

The intercondylar stud and the intercondylar gap together providestability and at the same time center the relative movement of thefemoral element and of the mobile plate. The intercondylar stud abutsthe extremities of the slot, thus limiting the amplitude of the movementduring extension.

The radius R″ of the sphere portion or of the cylinder portion form theintercondylar gap and can range from 14 mm to 30 mm, preferably 17 mm to25 mm.

It must be noted that the radii R, R′ and R″ are measured between theaxis of the cylinder generated by revolution or the center of thesphere, and the surface of friction of one part against the other.

Advantageously, said gap and condylar stud are spaced apart from eachother by a distance of 0 μm to 100 μm, preferably 10 μm to 100 μm, andeven more preferably 10 μm to 60 μm when the longitudinal axis of thefemoral element and the longitudinal axis of mobile plate form an angleof 0° to 60°.

Advantageously, R and R′ have a length of 22 mm to 38 mm, preferably 25mm to 35 mm.

The Applicant has noted that these special dimensions make it possibleto design knee prostheses for individuals of differing body mass whilepreventing the concentration of stresses on a precise area of theceramic material. In other words, these particular dimensions make itpossible to obtain the perfect congruency needed to obtain a prosthesisaccording to the invention, wherein the femoral element, the mobileplate and tibial plateau are made of massive ceramic.

Advantageously, said mobile plate has a perimeter smaller than theperimeter of the upper surface of the tibial plateau. Thischaracteristic enables the prosthesis to have a slightly pivotingmovement about the longitudinal axis of the mobile plate and the tibialplateau, this longitudinal axis essentially coinciding with thelongitudinal axis of the tibia. It must be noted that the longitudinalaxis of the mobile plate is essentially parallel, preferably perfectlyparallel, to the longitudinal axis of the tibial plateau.

In one advantageous embodiment, said tibial plateau comprises at leastthree stops forming a projection towards said mobile plate, said stopsmaking it possible to limit the movements of the mobile plate on thesurface of the tibial plateau.

The stops prevent uncontrolled translation of the mobile plate on thesurface of the tibial plateau. The stops therefore define a perimeter offree but controlled movement of the mobile plate relative to the tibialplateau. Thus, the movements permitted by the prosthesis are very closeto the movements of a natural knee.

Advantageously, the surface of mutual friction between said mobile plateand said tibial plateau is essentially plane. Preferably, the surface ofmutual friction between said mobile plate and said tibial plateau isperfectly plane. Thus, the sliding and/or translational movements of theplates relative to each other are almost devoid of friction orcompletely devoid of friction. Lacking rough surfaces, the risks of wearare limited. The movement of the knee is therefore more fluid and morenatural.

In one advantageous embodiment, said mobile plate can pivot on saidtibial plateau about an axis at an angle of +/−15°. In other words, thetransversal axis of the mobile plate can form an angle of 15° relativeto the transversal axis on the tibial plateau. It can noted byconvention that there is pivoting of +15° when the movement takes placein the clockwise sense and −15° when the movement takes place in theanticlockwise sense. This amplitude of movement is sufficient to ensurethe rotational movements of the natural knee, especially during flexionof the knee.

In one advantageous embodiment, the parts forming the prosthesisaccording to the invention have no holes and/or perforations. Thus, thefemoral element, the tibial plateau and the mobile plate have no holesor perforations. If we consider these parts to be made out of a samemassive ceramic material, it follows that the integrity of the ceramicis preserved by not making any hole or any perforation therein. Thisfurther contributes to the resistance of the prosthesis.

In one advantageous embodiment, each part forming the prosthesis is aunit part. Thus, the femoral element, the tibial plateau and the mobileplate are each formed by a unique part made out of a massive ceramicmaterial. On the contrary, none of these parts comprises an additionalsecondary part. Similarly, none of them is formed by an assembly of atleast two distinct parts. This further contributes to the resistance ofthe prosthesis and to the simplicity of design.

5. LIST OF FIGURES

Other features and characteristic of the invention shall appear moreclearly from the following description of a preferred embodiment, givenby way of a simple illustratory and non-exhaustive example and from theappended drawings of which:

FIG. 1 is a perspective or three-quarter, exploded view of theprosthesis according to the invention;

FIG. 2 is a sagittal section of the prosthesis according to theinvention when the longitudinal axis of the femoral element and thelongitudinal axis of the mobile plate form a zero (0°) angle;

FIG. 3 is a sagittal section of the prosthesis according to theinvention when the longitudinal axis of the femoral element and thelongitudinal axis of the mobile plate form an angle of 45°;

FIG. 4 is a sagittal section of the prosthesis according to theinvention when the longitudinal axis of the femoral element andlongitudinal axis of the mobile plate form an angle of 135°;

FIG. 5A is a top view of the tibial plateau and of the mobile plate whenthe transversal axis of the mobile plate and the transversal axis of thetibial plateau form an angle of +15°;

FIG. 5B is a top view of the tibial plateau and of the mobile plate whenthe transversal axis of the mobile plate and the transversal axis of thetibial plateau form an angle of −15°;

FIG. 5C is a top view of the tibial plateau and of the mobile plate whenthe transversal axis of the mobile plate and the transversal axis of thetibial plateau form a zero (0°) angle.

6. DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

The general principle of the invention relies on a total prosthesis fora knee in which the mobile plate, the femoral element and tibial plateauhave surfaces of mutual friction made out of one and the same ceramicmaterial. These surfaces of mutual friction are perfectly congruentbecause of the optimizing of the shape of the surfaces of mutualfriction and of the respective distance between the parts. Contrary tothe hitherto common and widespread view, ceramic can be used to obtain aparticularly resistant knee prosthesis.

This resistance is especially due to the perfect congruency of the partsrelative to each other and especially of the mobile plate relative tothe femoral element and the mobile plate relative to the tibial plateau.The perfect congruency of the mobile plate with the femoral elementprovides for optimum friction while generating a minimum of wear debris;its mobility reproduces the overall play of the joint. This play can besub-divided into two movements: that of the femoral element on themobile plate associated with the rotational mobility and a drawer-likemovement of the mobile plate on the surface of the tibial baseplate. Theentire innovation therefore prevents or at least restricts theprobabilities of repeat surgery on the knee, especially for young andphysical active patients. It furthermore improves the results of theknee prosthesis in terms of the comfort and stability. This perfectcongruency is obtained through studies on the part of the inventors,leading to the development of surfaces of mutual friction that preventthe creation of a concentration of mechanical stresses in theprosthesis. The congruency therefore prevents the phenomenon of abnormalwear of the ceramic material.

Referring to FIG. 1, we present a three-quarter and exploded view of theprosthesis according to the present invention.

As can be seen in FIG. 1, the prosthesis according to the inventioncomprises a femoral element 1, a mobile plate 2 and a tibial plateau 3fixed to an anchoring heel 4 that is to be mounted on a upper profiledend of a tibia.

The femoral element 1 has the general shape of a hollow partial shellexternally defining two condyles 11 appreciably reproducing the shape ofnatural condyles, the posterior floating part of which, however, withthe mobile plateau are cylinder portions and not irregular sphereportions. The condyles 11 could also be sphere portions. The condyles 11are separated by a slot that on the whole reproduces the naturalintercondylar gap 12. The intercondylar gap 12 has two appreciablyparallel lateral surfaces 13. The intercondylar gap 12 corresponds to acylinder portion coaxial with the cylinder portions forming the condyles11 but with a smaller radius. The intercondylar gap 12 can also take theform of a sphere portion, the center of which is on the axis ofrevolution of the cylinder portions forming the condyles 11 or on theaxis connecting the center of these spheres when the condyles are sphereportions.

Internally, the femoral element defines a femoral housing 16 that is toreceive the profiled lower extremity F1 of a femur F. The femoralelement 1 can have a protrusion on its anterior face 15 that mimics thetrochlea and is intended to cooperate with the patient's patella.

The mobile plate 2 is inserted between the femoral element 1 and thetibial plateau 3. The mobile plate has a lower surface 23 that isessentially or even perfectly plane. On its upper surface, the mobileplate 2 has two condylar bowls 21 separated by an intercondylar stud 24.The intercondylar stud has a surface 25, the shape of which correspondsto the gap 12 as well as two lateral surfaces 23, the shape of whichcorresponds to the lateral surfaces 13 of the gap. A slight intersticeenables the natural liquids to lubricate the joint formed by theprosthesis.

The condylar bowls 21 define areas or surfaces of mutual friction,respectively for the two condyles 11 of the femoral element 1.Advantageously, the bowls 31 and the condyles 11 have surfaces of mutualfriction of a perfectly congruent cylindrical shape. This significantlyreduces the friction and therefore the wear on these parts. The term“perfectly congruent” will be understood to mean that the parts fit inperfectly with each other and that they are spaced apart at a distanceof about 0 μm to 100 μm, preferably from 10 μm to 100 μm and even morepreferably from 10 μm and 60 μm, especially when the angle between thelongitudinal axis of the femoral element and of the mobile plate formsan angle of 0° to 75°.

As for the intercondylar stud 24, it is engaged in the intercondylar gap12 of the femoral element 1. The femoral element 1 abuts the surface 14in the intercondylar gap 12 so that the relative movement of the mobileplate 2 and of the femoral element 1 remains controlled in itsamplitude.

During the rotation of the femoral element 1 on the mobile plate 2, thecondyles 11 slip or slide in the condylar bowls 21 with theintercondylar stud 32 which moves in the intercondylar gap 12 that formsa sort of rail.

The mobile plate 2 can move freely in translation without any stress onthe tibial plateau 3, the lower surface 22 of the mobile plate and uppersurface 31 of the tibial plateau being both essentially or evenperfectly plane.

The tibial plateau has an upper surface 31 that is to cooperate with thelower surface 22 of the mobile plate 2. The mobile plate 3 has threestops 32 on its upper surface. These three stops 32 restrict thetranslational motion of the mobile plate 2. On its lower face, it has amassive rod made of alumina ceramic that is to be cemented in the upperextremity of a patient's prepared tibia.

Indeed, the mobile plate 2 herein fulfils the role of the meniscus. Itsperimeter is smaller than that of the tibial plateau 3 so that it canenter into movement within the limits of the space defined by the stops32. It can therefore slide in translation from front to rear and/orlaterally on the upper surface of the tibial plateau: this has theeffect of allowing further degrees of freedom in movement for thepatient and a movement very close to the natural movement of the knee.It can also pivot about its longitudinal axis, parallel to the axis ofthe tibial plateau.

The femoral element 1, the mobile plate 2 and the tibial plateau 3 areall three made out of ceramic Al₂O₃ according to the ISO-6474-1standard.

FIGS. 2, 3 and 4 are sagittal sections of the prosthesis assembledaccording to the invention. The references are identical with those ofFIG. 1. These sections are made in a sagittal plane spaced apart by adistance of about 20-27 mm from the median plane of the prosthesis.

As can be seen in FIG. 2, the longitudinal axis L of the tibial plateauand the longitudinal axis L′ of the femoral plate herein form a zero(0°) angle. In FIG. 3, the axis L and L′ form an angle α of about 45°.This corresponds approximately to the angle formed by the knee whenwalking. In FIG. 4, the angle α formed by the angles L and L′ is about135°. This enables the patient to walk up staircases or perform moderateexercise.

As can also be seen in FIG. 2, the condyles 11 are formed by cylinderportions with a radius R, the axis of revolution of which intersects theaxis L and L′ at the center O. It must be noted that the condyles couldbe sphere portions with a center O and a radius R′. Preferably, theradii R and R′ have a length of 22 mm to 38 mm, preferably 25 mm to 35mm. The radius R″ of the intercondylar gap cannot be shown in thesesections. The surface of the gap is herein concealed.

In the same way, FIGS. 5A to 5C present a top view of the relativemovement of the mobile plate 2 on the tibial plateau 3. As can be noted,the perimeter of the plate 2 is smaller than that of the plateau 3 sothat the mobile plate 2 can shift on the surface of the plateau 3. Thethree stops 32 define a perimeter or field of movement of the mobileplate 2. These top views are intended to illustrate the amplitude of thepermitted movement of the mobile plate relative to the tibial plateau.

As can be seen in FIGS. 5A and 5B, the plate 2 can pivot about itslongitudinal axis, parallel to that of the tibial plateau (not shown inthe figures). More specifically, the transversal axis of the tibialplateau 2 can form an angle β of about 15° relative to the transversalaxis T′ of the mobile plate 2.

By convention, an angle is denoted as being an angle of +15° when therotation is made in the clockwise sense (FIG. 5B) and −15° when therotation is made in the anticlockwise sense (FIG. 5C).

7. VARIANTS

Different variants of the invention can be envisaged. For example thecondyles and condylar bowls can both have the shape of cylinder portionswhile the intercondylar gap and the intercondylar stud take the form ofa sphere portion. Conversely, the condyles and condylar bowls can bothtake the shape of a sphere portion while the intercondylar gap andintercondylar stud take the shape of a cylinder portion. It is alsopossible for one condyle and its corresponding condylar bowl to take theshape of a sphere portion while the other will take the shape of acylinder portion.

In another variant, compatible with the variants listed here above, thetibial plateau is devoid of stops. In this case, the leg of the patientbeing operated on is held still by a splint while fibrous tissue isformed about the prosthesis to stabilize the replaced joint.

An exemplary embodiment of the present disclosure overcomes thedrawbacks of the prior art.

An exemplary embodiment provides a knee prosthesis that is moreresistant than presently used prostheses.

An exemplary embodiment implements a prosthesis of this kind that allowspatients to resume normal physical activity.

An exemplary embodiment proposes a prosthesis that limits inflammatoryreactions.

An exemplary embodiment proposes a prosthesis that enables there-forming of a fibrous tissue that is more resistant, and plays thenatural role of the knee ligaments by remodeling fibrous structuresunder mechanical stress.

An exemplary embodiment proposes a prosthesis, the joint or articularsurfaces (the surfaces of mutual friction) of which have improvedcongruency in a load-bearing area.

An exemplary embodiment proposes a prosthesis that enables greateramplitude of motion as compared with known knee prostheses.

Although the present disclosure has been described with reference to oneor more examples, workers skilled in the art will recognize that changesmay be made in form and detail without departing from the scope of thedisclosure and/or the appended claims.

What is claimed is:
 1. A total knee prosthesis to be implanted in ahuman patient, said prosthesis comprising: a femoral element having alongitudinal axis; a tibial plateau having a longitudinal axis; and amobile plate, said mobile plate being interposed between the femoralelement and the tibial plateau to form two joints with them wherein: a.said femoral element, mobile plate and tibial plateau are each formedand entirely constituted out of one and the same ceramic material, andsaid femoral element, mobile plate and tibial plateau do not include anyalloy with plastic material or metal material; b. the surfaces of mutualfriction of the femoral element with the mobile plate and the surfacesof mutual friction of the tibial plateau with the mobile plate areentirely constituted by said one and the same ceramic material; and c.the mobile plate comprises two condylar bowls, and the femoral elementcomprises two condyles, said condyles and said condylar bowls eachcomprising surfaces of mutual friction spaced apart from each other by adistance smaller than 100 μm when the longitudinal axes of said femoralelement and said tibial plateau form an angle of 0° to 75° and whereinsaid mobile plate comprises an intercondylar stud forming a projectiontoward said femoral element, said intercondylar stud beingmonolithically formed with the condylar bowls and having an uppersurface and two lateral surfaces, wherein said femoral element comprisesan intercondylar gap forming a rail that houses said intercondylar stud,said intercondylar gap having two parallel lateral surfaces, wherein theupper surface of the intercondylar stud corresponds to the shape of theintercondylar gap and the two lateral surfaces of the intercondylar studcorrespond to the ones of the parallel lateral surfaces of theintercondylar gap, forming three surfaces of mutual friction.
 2. Thetotal knee prosthesis according to claim 1, wherein said condylar bowlsand said condyles are spaced apart from one another by a distance ofless than 100 μm when the longitudinal axes of said femoral element andof said tibial plateau form an angle of 0° to 60°.
 3. The total kneeprosthesis according to claim 1, wherein the femoral element and thetibial plateau are mobile in rotation relative to each other on anangular range of flexion of 0° to 135°.
 4. The total knee prosthesisaccording to claim 1, wherein the surfaces of mutual friction of thecondyles and of the condylar bowls are cylinder portions generated byrevolution having a same radius.
 5. The total knee prosthesis accordingto claim 1, wherein said mobile plate has a perimeter smaller than theperimeter of an upper surface of the tibial plateau.
 6. The total kneeprosthesis according to claim 1, wherein said tibial plateau comprisesat least three stops forming a projection towards said mobile plate,said stops making it possible to limit movements of the mobile plate onthe surface of the tibial plateau.
 7. The total knee prosthesisaccording to claim 1, wherein said mobile plate can pivot on said tibialplateau such that a transversal axis of the mobile plate forms an angleup to +15° relative to a transversal axis of the tibial plateau whenmovement takes place in a clockwise sense and such that the transversalaxis of the mobile plate forms an angle of up to −15° relative to thetransversal axis of the tibial plateau when movement takes place in ananticlockwise sense.
 8. The total knee prosthesis according to claim 1,wherein a space between the surfaces of mutual friction of the mobileplate and of the tibial plateau is smaller than 100 μm.
 9. The totalknee prosthesis according to claim 1, wherein said ceramic material hasa thickness of 4 mm to 14 mm.
 10. The total knee prosthesis according toclaim 1, wherein said ceramic material is alumina Al₂O₃.